What are the consequences of a large pressure difference before and after the control valve?

The pressure difference before and after is greater than the design value: 1 The automatic control valve may not be able to close completely. 2 The unbalanced force on the valve increases, leading to control oscillations or sticking during the opening and closing of the valve. 3 Severe cases can lead to valve stem fracture and damage to the valve actuator.

I’m not quite sure what you mean – are you referring to a large pressure difference before and after the control valve during actual operation? How much is it? Control valves are designed to be controlled in conjunction with parameters such as flow rate or temperature. A large pressure difference before and after the control valve simply indicates that the pressure from the fuel oil source is high, meaning that the head capacity of the fuel oil pump has been set too high.

The pressure difference before and after the control valve is related to the pressure ahead of it as well as to the degree of opening of the valve. A two-seat valve has a stronger ability to resist an increase in pressure difference, whereas a single-seat valve has a weaker ability to do so. As the pressure difference across the valve increases, the single-seat valve may experience: 1. An increase in unbalanced forces, with an increased thrust on the valve stem when the valve is closed ； 2. Poor sealing, increased leakage ； 3. Oscillation occurs at low opening degrees.

The pressure difference before and after the valve is high, resulting in increased pressure loss in the pipes connected to the valve and higher energy consumption. At the same time, the requirements for valve components are also high ； When the pressure difference before and after the valve is large enough, the valve operates in a choked flow regime; at this point, the flow rate depends only on the degree of valve opening, and not on changes in the pressure difference before and after the valve, thereby **improving the stability of system control** ； For control engineering, as long as the right valve is selected, it doesn’t matter what the pressure difference before and after the valve is, nor is any adjustment necessary.

If the actual differential pressure is higher than that calculated for the valve, the opening degree will be smaller. However, the actual flow rate is related to the opening percentage (the flow area at this point) and the actual pressure drop. For example, in the case of liquid-phase pipelines with PIC control, the pressure drop across the valve at low opening degrees is actually relatively high. Valves of this type exhibit linear behavior when the opening degree is between 10-20%, but the increase in flow rate between 20-30% is smaller. At this point, although the PID control signal increases, the actual flow rate through the valve does not increase much; the valve is unable to keep up with changes in the setpoint, which results in poor accuracy (or slow response) of the control loop. But if it can still meet the process control accuracy (such as 5% of the set value), that’s acceptable. For process operation, what he is actually concerned about is the relationship between AO and flow rate (the opening degree is merely a bridge), while the valve’s flow rate is controlled by the changing pressure drop. PID parameters are set for a specific flow load; if the load changes significantly, these parameters may not meet the control accuracy requirements of the PID controller. If the pressure drop across the valve is currently greater than the resistance loss in the pipeline, I recommend tightening the manual valve downstream of the valve to force the opening degree of the control valve to over 40%, and then reset the PID parameters. If it is less than half of the pipeline pressure drop, the check valve effect is limited, as the valve opening increases rapidly once the process throughput is raised. The only option is to try to reduce the pressure at the source in order to increase the opening degree.

It’s not that serious either; at most, cavitation can cause damage to the control valve. In reality, such damage has never occurred. On several occasions, the process engineers reduced the differential pressure, and when calculating the control valve, it’s necessary to take these process factors into account by appropriately reducing the size of the valve stem.

Vibration: the pipeline vibrates, and the flow rate cannot be controlled

The pressure behind the valve is generally quite stable. If the pressure difference is large, it is likely that the pressure in front of the valve is high; at the same opening degree, a higher flow rate occurs